See, there is a story,
even life-breath yet,
in fallen things.
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Wasp gall on baby oak leaf, raided by a bird for the larva inside
Leaf with gall then fell to the ground
Fern Hollow, 14 May 2018 |
| I. | Leaf stalk shenanigans |
| II. | Catkins and camouflage |
| III. | Correction |
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Leaf Stalk Shenanigans
Ah, spring and early summer! Wildflowers blooming...birds building their nests...leaves falling...
Wait, what?
On a stroll recently at Winneshiek County's own Chattahoochie Park, I came across young green leaves of a
Populus sp. tree scattered on the ground. They hadn't dessicated much yet, which meant they had fallen very recently -- probably within the last 24 hours. Close inspection revealed that their petioles had been mysteriously severed.
Examining the trees from which these leaves had dropped, I found petiole "stumps" still attached to the trees' new shoots.
Here's a leaf whose petiole had been weakened but not quite broken.
Who or what is responsible for this? Plant pathogen, windy weather, insect feeding?
Well, if you look carefully at the image above, you can see the shed skin of a planthopper adhering to the petiole near the leaf blade. Planthoppers feed on plant juices, puncturing their host's tissue and sucking up the fluids inside. As you might imagine, this can cause local wilting or shriveling similar to the damage these
Populus leaves' petioles experienced. In fact, as I looked closely at the tree's branches, I discovered that there were planthopper nymphs hiding cryptically on the twigs and shoots. With their pointy rear ends oriented toward the twig tips, and their small bodies appressed closely to their perches, they could easily be mistaken for buds or leaf scars.
But I didn't yet feel comfortable indicting these little beasts. Perhaps their feeding had caused some of the leaves' petioles to break; but how could their piercing and sucking mouthparts result in petiole damage like this?
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| Damaged petiole of a prematurely shed Populus leaf, Chattahoochie Park, 20 May 2018 |
This leaf, it seemed, had been chewed to death: someone with opposing mandibles had cut out big chunks of its petiole, weakening the petiole and eventually causing it to break. A caterpillar seemed a likely culprit. However, my brief inspection of the
Populus branches revealed no moth or butterfly larvae whose jaws would be sizable enough to do this sort of damage. Though disappointing, this was no surprise; many foliage-feeding caterpillars actually hide on or near the ground during the day, ascending into the canopy at night to feed.
To test the idea that those little pointy-butt planthoppers had something to do with all this, I confined a few of them in a terrarium with cut branches from their host tree, keeping the branches fresh by immersing the cut ends in water. After maybe a week in there, the hoppers seemed alive and well, and at least one of them had even molted -- but none of the leaves had detached at the petiole.
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Ultimately, I don't know what caused the
Populus petioles to break. Tracking any kind of living thing often leads to more questions than answers, and that's part of what makes it so tantalizing. Of course, the other fun part is when you actually discover which creature left a particular sign you've been noticing! That happened to me just the other day, after a while spent trying to pin down the culprit in another fallen-leaf mystery...this one involving leaves of sugar maple.
The above leaves were all photographed in the past couple of weeks in parks and other public places around Decorah. Can you see how they all have really short petioles? As with the
Populus leaves, something had apparently been feeding on the petioles, causing them to break. In this case, the culprit had actually been feeding
inside the sugar maple petioles, hollowing them out as a result.
Cutting into one such petiole piece, I found frass (insect poop) inside.
About a year ago, while doing some research in connection with a similar leaf drop my parents noticed among some of the Norway maples in town, I came across the following passage in an entomology book. Reading it again now, I'm amazed at how precisely it describes the sign on sugar maple I've been seeing this spring.
Maple petiole borer (Caulocampus acericaulis) is a common insect associated with sugar maple in much of the eastern U.S. Larvae burrow into leaf petioles, which subsequently break near the blade. This produces a noticeable shedding of leaves in late May and early June. Larvae pupate in the soil and have one generation per year, with adults laying eggs in late April and May. [Emphasis added]
-- Garden Insects of North America by Whitney Cranshaw, p. 446 (1)
This had to be it! Now to actually find a larva.
Which seemed like the hard part...if they pupate in the soil, then they probably drop out of the petiole when it breaks, right? And even if they don't...how was I supposed to access the other end of the petiole, which was presumably still attached to the tree, way high up there in the canopy?
Whitney includes a photograph of a maple petiole borer larva in his text, though, which made me think...if the person who took that photograph could find a larva, then I can too. So, on a walk in sugar maple land the other day, I grabbed a low-hanging branch and started scanning it for still-attached petiole bases that were missing their leaf blades. And hey, whaddya know!
Hollowed out and everything!
I figured the larva had already dropped to the ground, but just in case, I decided to cut into this petiole stump...and, much to my excitement, the larva was still in there!
Examining the larva carefully, I found that it had six legs, opposing mandibles, and no prolegs...just what you'd expect for a sawfly larva. Some beetle larvae can have these characteristics too, but I'd seen enough to be satisfied with calling this a
Caulocampus sawfly
for now and leaving it at that (I've never had much luck rearing sawflies).
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Speaking of creatures that like to mess with leaf stalks...
Can you see the dead leaflets still attached to their petioles on this
Parthenocissus vine? I came across this puzzling sign last June, in a woods across the river in Wisconsin. The petioles had been nipped right where they joined the leaf blades, causing the blades to shrivel and turn brown but leaving the petioles green and succulent.
As it turned out, each little packet of shriveled leaflets had a caterpillar inside.
I kept the affected leaves in rearing jars, and eventually, the caterpillars finished feeding, pupated, and emerged as adults. Here's one of the resulting moths.
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| My, what big green eyes you've got! |
Bugguide users recognized this moth as belonging to the Tortricidae clan, a large family known informally as the "leafroller moths." Some tortricids specialize in particular plant hosts; others are generalists, feeding on many different plant species.
This May and June, I've been learning that the "petiole nipping" habit (as I like to call it) exhibited by the Parthenocissus caterpillar is actually quite widespread among certain moth larvae in our area. In each case, when a larva nips a petiole of its host plant, the affected leaf wilts or shrivels up, creating a cozy little home for the larva, who can be found hiding inside it. (Sometimes the larva nips the green plant stem rather than a leaf petiole, causing the whole shoot tip to wilt.) I believe the larvae shown here are all tortricids, though they may not all be different species. To photograph the larvae, I collected each shriveled-leaf shelter, brought it home, and carefully opened it enough to reveal the caterpillar inside.
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| Wild geranium, Geranium maculatum |
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| Virginia waterleaf, Hydrophyllum virginianum |
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| Cutleaf coneflower, Rudbeckia laciniata |
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| Native honeysuckle, Lonicera sp. |
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| Basswood, Tilia americana |
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| Pagoda dogwood, Cornus alternifolia |
And my personal favorite...on one of the so-called "false" Solomon's seals, Maianthemum sp.:
Hah! I bet you were expecting to see more than just a caterpillar butt there! Well, when you try to coax a tortricid larva out of its shelter, you've got a 50/50 chance that the end that comes out first will be the head end...and this time, it wasn't. I had to open the shelter up a bit more in order to get the mug shot...
:-)
Interestingly, in the basswood and dogwood examples, the petiole/stem has been gouged or hollowed out rather than nipped.
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| Petiole at base of leaf blade, basswood, Tilia americana (same leaf as the one shown above). A caterpillar gouged a hole in the petiole, causing the leaf blade to wilt. |
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| Shoot of pagoda dogwood, Cornus alternifolia, gouged out by a tortricid moth larva, causing the leaves to wilt (same group of leaves as the one shown above) |
The technique used to wound the plant is a bit different, but the result (wilted leaves) is the same -- as is the larval behavior that follows (hiding in the wilted leaves and feeding on them).
Another interesting variation on the nipping technique happens when a larva cuts all the way through the leaf petiole, causing the leaf blade (and hungry larva perched on it!) to fall to the ground. The leaf blade still dies, and the caterpillar still constructs a feeding shelter out of it -- but as the caterpillar feeds and grows, its limp and wilty leaf-home is resting on the ground rather than dangling from the host plant.
The following example is from wild bergamot,
Monarda fistulosa. I happened to see the leaf fall from the plant while I was sitting nearby.
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| Petiole base of wild bergamot leaf, freshly cut by a caterpillar |
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| The rest of the leaf, and the larva perched on it -- photographed only a few seconds after the leaf dropped from the plant. |
I stashed this leaf in a container I had handy at the moment, and by the time I got home the larva had already drawn together the leaf margins with silk, creating a tubelike feeding shelter.
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| Larva, removed from leaf wrap temporarily for photo shoot |
On June 13 -- after having grown to maturity on
Monarda leaves I'd fed it, and then undergoing a very brief pupal stage -- this critter completed its transformation into an adult moth.
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Adult of petiole-cutting caterpillar from wild bergamot, Monarda fistulosa
(It got a bit roughed up in the rearing container, hence the darker patch with scales worn off.) |
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| Adult, Monarda petiole-cutter |
And here's an adult of the
Hydrophyllum petiole-nipper:
The shoot-gouging caterpillars from dogwood haven't turned into pupae or adults yet, but when I checked in on them the other day, they seemed to have matured nicely in their wilted-leaf shelters. In fact, more than one of them had taken on a lovely pinkish-red color, as certain caterpillars often do shortly before they pupate.
It looks to me like at least four species of tortricids are represented in the preceding images. Why would so many different types of moths share this leaf-wilting habit? Why does it make sense to do this?
The "shelter hypothesis" I mentioned earlier is one possible reason: a wilted or shriveled leaf would seem an ideal hideaway, relatively safe from predators and parasitoids. Nipping the petiole could have chemical advantages for the caterpillar, too: a leaf damaged in this way is presumably unable to receive or manufacture defensive chemicals that could otherwise protect it from herbivory. Indeed, monarch caterpillars are known to nibble midveins or petioles of milkweed leaves in order to cut off the flow of unpalatable latex (2).
Reflecting on his latest sightings of clever behaviors in the animal world, a fellow naturalist recently told me he felt overwhelmed by the animals' sheer...competence. And isn't it so? The fittingness and precision of certain behaviors can be hard to believe. We are surrounded by beings who really know their stuff.
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Adult, Maianthemum leaf wilter
Photographed on June 26, 2018 |
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| Tortricids' eyes are pretty cool. |
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Catkins and Camouflage
In spring, oaks, poplars, and certain other trees present their flowers to the world in long, delicate, dangling structures known as catkins. Some of these catkins bear only male flowers, while others bear only female flowers. When the male catkins have done their job (shedding pollen), they fall from the tree en masse. This can create a mess for homeowners! Such was the case late this May in the front yard of a house on Broadway Street in Decorah.
These male catkins, dropped by a sturdy red oak tree, littered lawn, porch, roof, and gutter of the house. On the sidewalk, large bunches of the catkins had gathered themselves together like some feral breed of dust bunny.
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| Large indeed! |
Though likely bothersome from the homeowner's perspective, this windfall of catkins was also an opportunity to observe a certain creature that relies on the oak flowers for room and board. I first encountered this animal a few years ago at Phelps Park in Decorah. There, I learned to recognize the shelter it builds for itself: a small number of catkins drawn and woven together.
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Catkin-shelter constructed by a Chionodes caterpillar
Photographed in spring 2018 on the gravel driveway of local friends Liz and Daniel |
If you look closely at one of these catkin-shelters, you can see the silk used to hold it together...
…and if you begin to gently pull apart the catkins -- ideally over a piece of paper or other smooth surface -- a little tan-colored caterpillar may come tumbling out.
It's impressively well-camouflaged for its life among the oak flowers.
Sometimes, when you evict such a caterpillar from its shelter, it enters "crawl-away mode" and cannot easily be reintroduced to its former home. With a little effort, though, I convinced this individual to reenter its shelter, which I had left mostly intact.
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| Home again, home again, jiggity-rig |
Apparently these caterpillars -- belonging to a species in the genus Chionodes -- begin their lives feeding and sheltering in red oak catkins that are still attached to the tree. When the catkins drop, the caterpillars ride them to the ground, safely tucked inside. They continue feeding in their catkin-shelters, and eventually pupate there.
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| Pupa of Chionodes caterpillar in its catkin-shelter, Phelps Park, 16 June 2016 |
The adult is a subtly shaded moth with shimmery golden highlights.
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| Adult Chionodes moth, reared from red oak catkins, 14 June 2016 |
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Of course, it's not just oak flowers that harbor interesting critters. Do you recognize these catkins?
They belong to a tree in the genus Populus -- that is to say, a cottonwood, aspen, or poplar (depending on the species). Way back in late April, for two days in a row I watched black-capped chickadees picking around in catkins of this type at Chattahoochie Park. At least once I saw one chickadee dart out in pursuit of a flying insect; but the rest of the time the birds seemed focused on some creature that was staying put (hiding from them!) in the catkins.
Searching beneath the chickadee tree I found a few catkins the tree had already dropped. I picked through two or three of them myself, not seeing anything. Then, with one of these apparently unoccupied catkins in my hand, I happened to look away for a moment -- and when I looked back, I'll be darned if a cryptically-patterned larva hadn't just crawled out of the catkin and onto my hand! The power of its camouflage stunned me. Here's one such larva, photographed in a clump of fallen catkins I collected.
Now that I knew what I was looking for, I redoubled my search and found a number of these larvae in the tree's discarded catkins. At home in my bug studio, close examination revealed that the larvae had distinct, darkened head capsules but no obvious true legs or prologs -- just six little bumps where the true legs would be.
This suggested the creature wasn't a moth larva (order Lepidoptera). Indeed, when I posted photos to Bugguide, entomologist Terry Harrison ruled out Lepidoptera and wondered if my mystery larva might be some kind of beetle (Coleoptera) -- specifically, a weevil.
To find out, I stuffed some occupied catkins into a small rearing container with a bit of moistened peat at the bottom. Eventually the larvae left the catkins and worked their way into the peat. The catkins got all gross and moldy around this time, so I removed them, wiped the excess moisture from the inside of the rearing container, and started drumming my fingers.
A few weeks later, guess who emerged from the peat?
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| Adult weevil reared from fallen Populus catkins collected at Chattahoochie Park, Decorah. Photographed on 12 June 2018 |
Terry was right!
As it turns out, I'm not the first naturalist to notice birds foraging in Populus catkins and then identify the birds' quarry as a weevil. Dave Leatherman, author of a regular "Hungry Birds" feature in Colorado Birds, made similar observations in Colorado, describing the sought-after catkin dweller as "a whitish beetle larva...[resembling] a small grain of cooked rice" and determining this animal to be a weevil in the genus Dorytomus (3). After noticing "warblers, vireos, kinglets, orioles, Red-winged Blackbirds, House Finches, grosbeaks, and tanagers all vigorously seeking" the larvae in cottonwood catkins, Dave wrote, "I propose that the catkin-infesting larvae of Dorytomus weevils are an underappreciated source of food for many bird species in spring migration." (3)
Rearing this animal from larva to adult in early spring made me wonder about the rest of its life cycle. Once the adult beetles emerge, what happens next? Do they mate right away and lay eggs? Because if they do, then the eggs -- presumably deposited somewhere high up in the host trees -- would have to make it all through the spring, summer, and fall, and then tough out our long Midwestern winter, before finally hatching next April. Another possibility is that the adults survive until fall (or even next spring?), waiting until then to lay their eggs. In the end, I don't know -- but it sure would be interesting to find out.
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Correction
In my April 15 post (
Sun and water, part 1), I wrote about certain flies in the family Heleomyzidae, which I'd been seeing in woods around Decorah. Here's a photo of one to jog your memory.
This one sure seemed to be basking in the sun. Given that Bugguide lists "sun flies" as a common name for the Heleomyzidae -- and that the "heleo" in there seemed clearly to refer to "sun" -- I waxed philosophical about my tree trunk fly:
Worshipping the sun god, eh?
Helios was, for Greeks, that god.
Clearly, then, it's fitting that
this family of flies bears his name--
Heleomyzidae!
(Hee-lee-oh-MY-zih-dee.)
The flies of Helios.
The sun flies!
But, as it turns out, I was mistaken...
Frequent Bugguide contributor John Carr, who helped identify some of my early spring flies as heleomyzids, pointed out a glaring (but, I dare say, understandable) error in my etymology. Being much better versed in Greek than I, John wrote, "Greek heleos refers to marsh, helios refers to sun." One letter makes all the difference!
Embarrassed at my gaffe, I started reading up more on my Greek, and discovered something that helped me feel better: this isn't the first time confusion has arisen about
heleos and
helios in the context of a scientific name. Check out this 1979 note (4) in regard to a genus of pitcher plants.
Interesting stuff (at least if you're into etymology like me!).
But getting back to heleomyzids... After receiving John's correction, I reflected a bit more on the research I'd done for "Sun and water, part 1." In retrospect, I realized the science I'd come across seemed to say little or nothing concrete about a special relationship between heleomyzids and sunlight. And here's the kicker: it didn't really mention any strong tie between heleomyzids and marshland, either!
So what do we actually know about the Heleomyzidae? Let's consult entomologist P. Skidmore, who writes about a subgroup of these flies.
Heleomyzines are primarily necrophagous ["death-eating" --JV], some breeding only in carrion. More often however they appear to breed in excrement of omnivorous or carnivorous birds and mammals. Most have a marked preference for breeding in shaded, or entirely dark, situations, so that many breed only in caves or mammal burrows. (...) Whilst Heleomyza species sometimes breed in sunlit places such as guano heaps below bird-cliffs, or chicken manure dumps outside battery-farms, they more often inhabit sheded cess-pits, cave entrances and old buildings (...). Some heleomyzines are very tolerant of low temperatures. Thus, Heleomyza borealis is one of the very few acalypterate flies which breed commonly in the High Arctic. (5)
Here are some more tidbits from the literature about a few particular species in the family.
"The larva [of Scoliocentra brachypterna] develops in birds' nests and with all probability in bat guano (imagines are known from several European caves) and in cat carrion." (6) [emphasis added]
"[Gymnomus caucasicus is] an alpine species, probably endemic for Caucasus Mts. Larvae possibly develop in carcasses of small mammals (rodents), where imagines have been collected (April to May)." (7) [emphasis added]
"Adults of Pseudoleria crassata Garrett were found around burrows of woodchucks, Marmota monax (L.), and eastern chipmunk, Tamias striatus (L.), and in the nesting cavities of the bank swallow, Riparia riparia (L.)." (8) [emphasis added]
Incidentally, Polish researcher Andrzej Woźnica -- who wrote the first and second quotes given above -- also volunteers at Bugguide, the website I use to share my own insect studies. When I posted the following picture of a fly on skunk cabbage, which also appeared in "Sun and water, part 1," Andrzej identified the critter as...guess what?
Andrzej also recently tracked down a couple papers by C. Fallen, the Swedish entomologist who gave these flies their family name way back in 1820. In an email to me, Andrzej wrote:
And that's pretty much all there is to it. Unfortunately, Fallen never told us
(and thus the family Heleomyzidae) deserved to be named after marshes. I'd like to think there was a good reason, and he just neglected to mention it...but who knows?
Thanks to Terry Harrison, Andrzej Woźnica, John Carr, and other Bugguide users for their help identifying some of the animals included in this post.
1. Cranshaw, W. 2004. Garden insects of North America. Princeton University Press: Princeton and Oxford.
